1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) mobile communication system and a method of controlling intersystem hand-off in such a CDMA mobile communication system, and more particularly to a method of controlling intersystem hand-off in a mobile communication system which employs a CDMA communication process.
2. Description of the Related Art
According to a conventional CDMA communication process based on IS-95 standards provided for by TIA (Telecommunication Industry Association) of the USA, a mobile station measures from time to time the strengths of pilot signals transmitted from the base station of a cell with which the mobile station is communicating and the base station of a nearby cell.
A network to which the mobile station is connected informs, in advance, the mobile station of information including a pilot signal strength condition indicating the availability of a cell and a pilot signal strength condition indicating the unavailability of a cell. The mobile station checks the measured pilot signal strengths against the supplied pilot signal strength conditions, and reports a pilot signal strength measurement message (hereinafter referred to as xe2x80x9cmeasurement messagexe2x80x9d) to the network when the measured pilot signal strengths undergo a state change.
Based on the measurement message from the mobile station, the network determines an added cell and a deleted cell, and performs hand-off control on the channel in use. Cells refer to regions which are managed by the network. The above CDMA communication process is disclosed in Japanese laid-open patent publications Nos. 10-336723 and 11-122654.
According to the above conventional CDMA communication process, if another CDMA communication system or an analog system exists adjacent to the CDMA mobile communication system in question, then a hand-off occurs to the adjacent system in a system boundary. In the case where a cell covered by the other system and a cell covered by the CDMA mobile communication system in question are too close to each other or a pilot signal strength of the other system is stronger, if an intersystem handoff is executed based solely on the report from the mobile station, then the hand-off is carried out too early, and the boundary cell becomes small.
Furthermore, according to the above conventional CDMA communication process, in the case where a cell covered by the CDMA mobile communication system in question and a cell covered by the other system are close to each other, if an intersystem handoff is executed based on the report from the mobile station, then a pilot signal strength of the cell covered by the CDMA mobile communication system in question occasionally appears strong immediately after the mobile station has moved to the cell covered by the other system. Consequently, the mobile station reports a measurement message, causing a hand-off to be repeated between the systems. Since an intersystem hand-off is a hard hand-off, it inevitably results in a short break that disrupts the speech quality of the call.
It is therefore an object of the present invention to provide a CDMA mobile communication system capable of solving the conventional problems and reducing short breaks due to hard hand-offs for thereby improving the speech quality of calls, and a method of controlling intersystem hand-off in such a CDMA mobile communication system.
A CDMA mobile communication system according to the present invention has a plurality of base stations, a base station controller for controlling the base stations, and a mobile station for reporting the signal strengths of pilot signals from the base stations to the base station to which the mobile station is connected. The base station controller comprises storage means, detecting means, changing means, and executing means.
The storage means stores margin values associated with respective cells as regions managed by base stations of an adjacent other system, which margin values are to be subtracted from the signal strengths of pilot signals of the base stations of the adjacent other system. The detecting means detects information contained in a report from the mobile station and representing the signal strength of the pilot signal from the base station. If the signal strength of the pilot signal from the base station is detected by the detecting means, the changing means subtracts a corresponding margin value stored in the storage means from the detected signal strength of the pilot signal for thereby changing the signal strength of the pilot signal. The executing means executes an intersystem hand-off to the other system if the signal strength of the pilot signal changed by the changing means exceeds a predetermined threshold value.
According to the present invention, there is also provided a method of controlling an intersystem hand-off to switch a mobile station connected to a base station of its own system to a base station of an adjacent other system. The method comprises the steps of detecting information contained in a report from the mobile station and representing the signal strength of a pilot signal from the base station, and if the signal strength of the pilot signal from the base station is detected, subtracting a corresponding margin value from the detected signal strength of the pilot signal for thereby changing the detected signal strength of the pilot signal, and executing an intersystem hand-off to the other system if the changed signal strength of the pilot signal exceeds a predetermined threshold value.
In the case where there is a movable system adjacent to the CDMA mobile communication system, as described above, the radius of a cell at a system boundary is not reduced, and an intersystem hand-off is prevented from being repeated when the mobile station is to move between the systems. Therefore, short breaks due to unwanted hard hand-offs are reduced, making it possible to improve the speech quality of calls.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.